A scanning optical apparatus for use in an electrophotographic image forming apparatus converts a beam of light emitted from a light source into a dot-like image to be formed on a target surface of a photoconductor drum or the like and scans the target surface in a direction of an axis of the photoconductor drum (main scanning direction) with the dot-like image. The scanning optical apparatus includes a deflector which deflect a beam of light in the main scanning direction, an illumination optical system provided upstream of the deflector, and a scanning optical system provided downstream of the deflector. The illumination optical system focuses the beam of light near the deflector in a sub-scanning direction and makes the same into a parallel or nearly parallel beam in the main scanning direction. On the other hand, the scanning optical system serves to convert the beam of light directed from the deflector, into a dot-like image on the target surface to be scanned.
As the ambient temperature of the scanning optical apparatus (the temperature of the surrounding medium which comes into contact with the apparatus) changes, the dimensions of various parts thereof as well as the properties of various optical elements thereof will change, and the focal point would disadvantageously be shifted forward or backward from the target surface. This problem would be conspicuous when the illumination optical system includes a lens made of plastic (resin) which may be adopted in view of cost reduction. In order to address this problem, the illumination optical system may include a refractive surface and a diffractive surface so that an undesirable shift of the focal point (image plane shift) due to change in temperature is suppressed. For example, it has been assumed to be preferable that a ratio of refractive power to diffractive power of the illumination optical system in the main scanning direction is in a range of 1.437 to 2.669, and that a ratio of refractive power to diffractive power of the illumination optical system in the sub-scanning direction is in a range of 1.437 to 2.669.